I decided on the colours beforehand and pre-mixed four colour/value chains, two for each horse: one to use for the light and one to represent the shade. Despite this precaution it was still tricky disentangling all those legs.
The legs presented the same conundrum as trees against the sky in landscape painting: do you paint the branches first or the sky first? The legs or the pasture?
There is no right answer, of course. I did a combination of both.